per Boyd Allen...
In the typical target chamber and some factory chambers, starting from the part that is designed to accommodate the case neck (For example, in the case of a .262 neck 6PPC this is a cylindrical section that is nominally .262 ID.) forward of this, the ID of the chamber funnels down (typically at a 45 degree angle) to just over bullet diameter where a new cylinder begins. This is the so called the freebore. My 6PPC reamer cuts a chamber that has a .060" long freebore that is a few ten thousandths over .243". At the front of the freebore the rifling starts. This is where the lands ramp up from zero to to full height and the groove diameter begins. In this section, sometimes referred to the as the leade, the typical taper of the rifling, referred to as the throat or leade angle, (again, for a typical target chamber) is 1 1/2 degrees. Zero freebore means that the leade begins right at next to the chamfer at the front of the neck part of the chamber. in this chamber drawing, the throat or freebore length is .105.
http://www.the-long-family.com/image...kley%20new.jpg
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